To increase the packing density in integrated circuits, so-called oxide edge-bounded bipolar transistors are being used more and more. Described as oxide edge-bounded bipolar transistor is a bipolar transistor whose emitter and base are bounded by an oxide edge at least on one side. The bipolar transistors have lower capacitances.
For the production, the oxide edge is produced on a substrate. After forming the base by ion implantation, a doped polysilicon layer, from which the emitter is produced by outdiffusion, is produced on the surface of the substrate and the oxide edge. Before the polysilicon layer is deposited, the surface of the substrate has to be pretreated, for example, by an HF dip. This process results in thinning of the base at the oxide edge. In the case of very shallow emitter-base profiles, in particular, this results in reduced emitter-collector yields and emitter-collector breakdown voltages in the finished bipolar transistor.
Since bipolar transistors which are not oxide edge-bounded have hitherto generally been used in bipolar technology, no solution to this problem is to be found in the prior art. Known bipolar transistors, such as are described, for example, in European reference EP-A-0 170 250, have an active transistor zone which is defined by the geometry of the base connection disposed at the surface of the substrate. An inactive base which surrounds both the active base and the emitter in an annular manner is formed in the substrate by outdiffusion from the base connection region. The problem of a reduced emitter-collector breakdown voltage does not therefore occur in these bipolar transistors.